Student's guide to diseases of the eye / by Edward Nettleship ; with a chapter on examination for color perception by William Thomson.

Nettleship, Edward, 1845-1913.

Date:: 1883

Credit: Student's guide to diseases of the eye / by Edward Nettleship ; with a chapter on examination for color perception by William Thomson. Source: Wellcome Collection.

Provider: This material has been provided by the Francis A. Countway Library of Medicine, through the Medical Heritage Library. The original may be consulted at the Francis A. Countway Library of Medicine, Harvard Medical School.

89/448 (page 75)

$determined with great accuracy {determination of the refrac- tion by the ophthalmoscope). In E., as already stated at pp. 64, 65, the erect image can be seen only if the observer be near to the patient, and also completely relax his accom- modation ; for, in experiment d. there described, when the head was withdrawn from the lens the magnifying power appeared to increase, whilst the field of view and illumina- tion rapidly diminished. The same occurs with the eye, but in a much greater degree, and hence in E. no useful view can be gained except near to the eye. In H., where the retina is within the focus of the lens- system, the erect image is seen when close to the patient's eye only by an effort of accommodation in the observer, just as in the same experiment when the lens was within its focal length from the page (p. 64, c). And as in that experiment the print was also seen easily, even when the head was withdrawn, so in H. the erect image is seen at a distance as well as close to the patient. If now the observer, instead of increasing the convexity of his crystalline, place a convex lens of equivalent power behind his ophthalmoscope mirror, this lens will be a measure of the patient's H., i. e., it will be the lens which, when the patient's accommodation is in abeyance, will be needed to bring parallel rays to a focus on his retina. If a higher lens be used, the result will be the same as when in the experiment the convex lens was removed beyond its focal length from the print; the fundus will be more or less blurred. . Hence to measure H.: (1) the accommodation of both patient and observer must be fully relaxed (usually by atropine in the patient and by voluntary effort in the ob- server) ; (2) The observer must go as close as possible to the patient; (3) he must then place convex lenses behind his mirror, beginning at the weakest and increasing the strength till the highest is reached with which the details$